As is known in the art, high dynamic range (HDR) video is starting to become available. HDR video has a dynamic range, i.e. the ratio between the brightest and darkest parts of the image, of 10000:1 or more. Dynamic range is sometimes expressed as “stops” which is logarithm to the base 2 of the dynamic range. A dynamic range of 10000:1 therefore equates to 13.29 stops. The best modern cameras can capture a dynamic range of 13.5 stops and this is improving as technology develops.
Conventional televisions (and computer displays) have a restricted dynamic range of about 100:1. This is sometimes referred to as standard dynamic range (SDR).
HDR video provides a subjectively improved viewing experience. It is sometime described as an increased sense of “being there” or alternatively as providing a more “immersive” experience. For this reason many producers of video would like to produce HDR video rather than SDR video. Furthermore since the industry worldwide is moving to HDR video, productions are already being made with high dynamic range, so that they are more likely to retain their value in a future HDR world.
Various attempts have been made to convert between HDR video signals and signals usable by devices using lower dynamic ranges (for simplicity referred to as standard dynamic range (SDR)). One such approach is to modify an opto electronic transfer function (OETF).
FIG. 1 shows an example system in which a modified OETF may be used to attempt to provide such conversion. An OETF is a function defining conversion of a brightness value from a camera to a “voltage” signal value for subsequent processing. For many years, a power law with exponent 0.5 (i.e. square root) has ubiquitously been used in cameras to convert from luminance to voltage. This opto-electronic transfer function (OETF) is defined in standard ITU Recommendation BT.709 (hereafter “Rec 709”) as:
  V  =      {                                        4.5            ⁢            L                                                              for              ⁢                                                          ⁢              0                        ≤            L            <            0.018                                                                          1.099              ⁢                              L                0.45                                      -            0.099                                                              for              ⁢                                                          ⁢              0.018                        ≤            L            ≤            1                              
where:
L is luminance of the image 0≤L≤1
V is the corresponding electrical signal Note that although the Rec 709 characteristic is defined in terms of the power 0.45, overall, including the linear potion of the characteristic, the characteristic is closely approximated by a pure power law with exponent 0.5.
Combined with a display gamma of 2.4 this gives an overall system gamma of 1.2. This deliberate overall system non-linearity is designed to compensate for the subjective effects of viewing pictures in a dark surround and at relatively low brightness. This compensation is sometimes known as “rendering intent”. The power law of approximately 0.5 is specified in Rec 709 and the display gamma of 2.4 is specified in ITU Recommendation BT.1886 (hereafter Rec 1886). Whilst the above processing performs well in many systems improvements are desirable for signals with extended dynamic range.
The arrangement shown in FIG. 1 comprises an HDR OETF 10 arranged to convert linear light from a scene into RGB signals. This will typically be provided in a camera. The RGB signals may be converted to YCbCr signals in a converter 12 for transmission and then converted from YCbCr back to RGB at converters 14 and 16 at a receiver. The RGB signals may then be provided to either an HDR display or SDR display. If the receiver is an HDR display then it will display the full dynamic range of the signal using the HDR EOTF 18 to accurately represent the original signal created by the HDR OETF. However, if the SDR display is used, the EOTF 20 within that display is unable to present the full dynamic range and so will necessarily provide some approximation to the appropriate luminance level for the upper luminance values of the signal. The way in which a standard dynamic range display approximates an HDR signal depends upon the relationship between the HDR OETF used at the transmitter side and the standard dynamic range EOTF used at the receiver side.
FIG. 2 shows various modifications to OETFs including the OETF of Rec 709 for comparison. These include a known “knee” arrangement favoured by camera makers who modify the OETF by adding a third section near white, by using a “knee”, to increase dynamic range and avoid clipping the signal. Also shown is a known “perceptual quantizer” arrangement. Lastly, a proposed arrangement using a curve that includes a power law portion and a log law portion is also shown. The way in which an SDR display using the matched Rec 1886 EOTF represents images produced using one of the HDR OETF depends upon the OETF selected. In the example of the Knee function, the OETF is exactly the same as the Rec 709 for most of the curve and any departs therefrom for upper luminance values. The effect for upper luminance values at an SDR receiver will be some inaccuracy.
The above described conversions consider the ability to present an HDR signal on an SDR display.
However, these conversions do not consider a further need to convert signals produced for one display such that they may be appropriately presented on a different display. Such a conversion may be needed, we have appreciated, even between HDR signals produced for one display so that they are usable on a different display. Conversions for providing appropriate rendering on different displays will depend upon the way in which a signal was produced and the way a target display renders the signal.